1. Field of Invention
The invention relates to a sensing structure, and in particular, to a capacitance sensing structure.
2. Related Art
With the enhancement of the semiconductor manufacturing technology, micro-electromechanical devices applied to a micro-electromechanical system (MEMS) are commonly manufactured using the complementary metal-oxide semiconductor (CMOS) technology.
The micro-electromechanical device manufactured according to the prior art has a suspended conductive structure serving as a sensing unit, which receives an external action to move the suspended conductive structure relative to a fixed conductive structure. A variation between the suspended conductive structure and the fixed conductive structure may be sensed so that a sensed value may be calculated. In the following example, a capacitive micro-accelerometer senses a variation of capacitance to derive a value of acceleration. The micro-accelerometers may be classified into an out-of-plane sensing mechanism and an in-plane sensing mechanism for respectively sensing a vertical variation and a lateral variation according to the structure design.
Referring to FIG. 1, a conventional out-of-plane capacitive micro-accelerometer 1 includes a proof mass 10, a flexible portion 11, a first comb-shaped electrode 12 and a second comb-shaped electrode 13. The capacitive micro-accelerometer 1 is manufactured by a CMOS manufacturing process of stacking a plurality of conductive layers and a plurality of dielectric layers alternately, and patterning the conductive layers to form longitudinal first comb-shaped electrodes 12 and longitudinal second comb-shaped electrodes 13, which serve as a sensing structure during the stacking process. Finally, the dielectric layers in some regions are removed by way of etching so that the first comb-shaped electrodes 12 and the second comb-shaped electrodes 13 are suspended. The proof mass 10 is connected with a fixing end 14 through the flexible portion 11. When no external force is applied, the proof mass 10 is at a stationary position, the first comb-shaped electrodes 12 are connected with the proof mass 10, the second comb-shaped electrodes 13 are disposed corresponding to the first comb-shaped electrodes 12, and the first comb-shaped electrodes 12 are aligned with the second comb-shaped electrodes 13 through a matching frame 15.
As shown in FIG. 2, when an external force is applied to the proof mass 10 to shift the proof mass 10 in a Z-axis direction, the sensing area between the first comb-shaped electrode 12 and the second comb-shaped electrode 13 varies. Thus, the capacitance variation between the comb-shaped electrodes 12 and 13, i.e. capacitance variation between capacitors C1 and C2, can be measured to derive the acceleration value.
However, because the comb-shaped electrodes 12 and 13 have the longitudinal comb-shaped structures, and the thickness of each conductive layer formed in the CMOS manufacturing process is only several hundreds angstroms (Å), the area for sensing the capacitance variation between the first comb-shaped electrode 12 and the second comb-shaped electrode 13 is very small. Thus, the capacitance variation is smaller and may be hidden in the parasitical capacitance, thereby decreasing the sensing sensitivity. Consequently, a capacitance sensing circuit, which is complicated and precise, has to be adopted to detect the capacitance variation. In addition, the longitudinal comb-shaped electrodes 12 and 13 may be deformed due to the residual stresses produced in the manufacturing processes and the insufficient rigidity thereof, thereby influencing the resolution of sensing the capacitance variation.
In addition, since the conventional structure does not provide a test electrode and a mechanical limit stop, the sensing structure has to be operated and tested carefully after being manufactured. When the exceeding acceleration variation is inputted, the sensing structures contact each other to form the short-circuited condition, and the unrecoverable functional damage may be caused. In addition, since a feedback control circuit is also designed without the consideration of the test electrode, the precision of the sensed value and thus the sensing range are also influenced when the capacitance variation is increased non-linearly. If the test electrode has to be disposed additionally, the manufacturing process and cost for the test electrode are needed and the circuit layout becomes more complicated.
Therefore, it is an important subject to provide a capacitance sensing structure capable of overcoming the above-mentioned problems.